1. Field of the Invention
The present invention is related to an apparatus for frequency synchronization and a method for the same, and more particularly, to an apparatus and method that are used to obtain the pilot tones and evaluate the frequency offset and time offset for frequency synchronization.
2. Description of Related Art
Conventionally, in a wireless communication system with a pilot channel, a mobile station must use the pilot channel to synchronize with the base station so as to establish a connection for data transfer. In time division multiple access (TDMA) systems, such as GSM system, a frequency correction channel (FCCH) is used for mobile stations to perform time and frequency synchronization. When the mobile stations are activated or performing a handoff process, they must use the FCCH to perform time and frequency synchronization. Only after the synchronization is performed can the mobile station establish a connection to the base station for data transfer.
Since the FCCH appears rapidly in data slots, the mobile station needs to detect the existence of FCCH to keep or establish the connection to the base station during a practical operation, especially during data transmission over a bad wireless channel, for example, which suffers from severe noise interference or has a frequency offset.
Reference is made to FIG. 1, which is a block diagram of a conventional frequency synchronization apparatus. The frequency synchronization apparatus includes a normalizer 100, a frequency shifter 110, a low-pass filter 120, a phase measuring circuit 130, a phase differentiator 140, a sum/dump device 150, a frequency offset estimation circuit 160 and a quality factor estimation circuit 170.
Therein, the normalizer 100 is used to normalize the received signals to restrict them within a predefined range. The normalizer 100 can be realized by using a lookup table. After normalization, the frequency shifter 110 moves the central frequency of the received signals to the baseband.
After filtering out a noise portion via the low-pass filter 120, the phase measuring circuit 130 extracts the phases of the received signals. After that, the phase differentiator 140 compares the phases of the signals to obtain phase offsets between the signals. After adding up the phase offsets via the sum/dump device 150, the frequency offset estimation circuit 160 uses the summation result to estimate the frequency offset. Then, the quality factor estimation circuit 170 uses the frequency offset to estimate a quality factor to determine whether the frequency offset is correct or not.
However, since the normalizer 100 of the conventional frequency synchronization apparatus is implemented by using a lookup table, the conventional frequency synchronization apparatus must have a memory with a great capacity to store the lookup table. Hence, this kind of conventional frequency synchronization apparatus is very expensive and impractical.
Reference is made to FIG. 2, which is a flow chart for illustrating the operation of another conventional frequency synchronization apparatus. As shown in the figure, the frequency synchronization apparatus first uses the first adaptive band-pass filter to filter the baseband signal Xn to produce the signal Yn (S201). The frequency synchronization apparatus can adjust the poles of the second adaptive band-pass filter via reference to the signal Yn (S202). Thereby, the frequency synchronization apparatus can make the second adaptive band-pass filter have a central frequency that is the same as the reference frequency provided by the FCCH. In addition, the frequency synchronization apparatus can also adjust the gain via reference to the signal Yn (S203-S205). Then, the frequency synchronization apparatus determines whether the frequency correction burst is received or not (S206) and updates the timer (S207).
Furthermore, the baseband signal Xn is first stored in the signal register (S208). After finishing adjusting the second adaptive band-pass filter, the frequency synchronization apparatus uses the second adaptive band-pass filter to filter the baseband signal Xn (S210). After that, the frequency synchronization apparatus uses the frequency offset estimation circuit to obtain the frequency offset (S209). Thereby, this kind of frequency synchronization apparatus can completely abstract the frequency signal provided by the FCCH to estimate the frequency offset of the system.
However, since this frequency synchronization apparatus uses adaptive filters to process the received signals to estimate the frequency offset and perform time-slot alignment, it causes some potential problems. For example, adjusting the parameters of the adaptive filter causes a time delay. Moreover, during adjustment of the adaptive filter, the values of the parameters may exceed a limit and make the frequency synchronization apparatus no longer able to detect the FCCH.